A protester holding a picture of Chinese writer Liu Xiaobo marches to the US consulate in Hong Kong, November 13. Photograph: Mike Clarke/AFP/Getty Images

One of China's leading dissidents has been charged with "inciting subversion", and faces a possible 15-year jail sentence, amid growing international outrage over his detention and forthcoming trial.

Liu Xiaobo was one of 300 democratic activists in China to author a bold call for constitutional reform last December. The manifesto was published under the name Charter 08, and called for greater freedom of expression, multi-party elections and independent courts. Seen as a figurehead for the movement, Liu was taken into detention shortly before the document was published online. Then, in June, he was formally arrested on suspicion of incitement to subvert state power.

In the latest development – which came on International Human Rights Day, a year and a day after the charter's publication – officials told Liu's lawyer they would charge him. He will almost certainly be convicted and sentenced to jail, say experts, probably within weeks.

"The timing is not coincidental," said Joshua Rosenzweig of the Dui Hua Foundation, which supports political prisoners. "It draws attention away from commemorating the document and says: 'Look, you want to talk about Charter 08? This is what it gets you.' "

Professor Perry Link, of Princeton University, New Jersey, who translated Charter 08 into English, said: "He must have known that he was running a risk of becoming the regime's target."

Last week 165 of the original signatories on the mainland issued a further letter, headed: "We Are Willing to Share Responsibility with Liu Xiaobo".

Liu's detention had already prompted an international outcry. Nobel laureates including Vaclav Havel, Wole Soyinka and Nadine Gordimer have called for his release. US officials are believed to have raised his name during President Barack Obama's visit to Beijing last month. But the Chinese authorities seem unruffled.

"I think the message to the outside world is, it doesn't really matter to the government how this case is viewed by the international community. It can do whatever it wants," Xu Youyu, a fellow signatory and philosophy professor, told the New York Times.

Liu, 53, has long been a thorn in the side of the authorities. In 1989 he was jailed for his role in the pro-reform protests in Tiananmen Square. In the 1990s he was sent to a re-education camp. But after release he repeatedly turned down opportunities to go abroad. Instead he continued to write "interesting, engaging and relevant essays", said Nicholas Bequelin, a senior Asia researcher at Human Rights Watch, commanding respect from the younger generation as well as his peers.

"Having five people follow you when you go for a coffee with a journalist, or people cutting off your phone – it's not easy. He has shown extraordinary strength of character," said Bequelin.

The author's wife, Liu Xia, described him as calm and gentle but "persistent in his pursuit of life".

The authorities appeared to tolerate his writings, despite obvious irritation. Then came Charter 08. As several signatories have pointed out, the statement – inspired by Czechoslovakia's Charter 77 – mostly calls for the implementation of commitments in the Chinese constitution. And Liu had often written on sensitive subjects, challenging Chinese policy on Tibet. Most believe the difference this time was the charter's wide-ranging support: drawn not just from activists but lawyers, scholars and even farmers.

"Probably the most worrying thing to the authorities was the broad coalition of people who decided to put their name on it," said Bequelin. "It was the organisation [that concerned them]; it was across different social groups and across the country. That's really one of the red lines for the party."

Scores of signatories were briefly detained and interrogated or harassed. Several have been penalised at work – losing the right to publish or, in one case, being exiled to a university at the other end of China. But only Liu has been charged. Supporters believe that reflects his high profile. Knock him down, and who will dare stand in his place?

"If they have their way, Liu will be converted from a champion of Charter 08 into an instrument of its repression," wrote Mo Shaoping, a lawyer and fellow signatory who has worked on Liu's case, earlier this year.

To many, it is hard to understand why the charter so alarmed authorities. Thousands signed up after its publication. But it is not clear how many of those were on the mainland; and the document was quickly wiped from internet sites in China. There is no sign that it had mass appeal – still less political impact.

The same might, of course, have been said for the document which inspired it. The path to 1989's Velvet Revolution was long. Presenting a human rights prize in honour of Liu earlier this year, Havel, the Charter 77 founder and former president of the Czech Republic, said: "One may never reckon with success; one may never reckon with the situation changing tomorrow, the day after tomorrow, or in 10 years. Perhaps it will not. However, we found that it was possible to change the situation after all, and those who were mocked as being Don Quixotes … may in the end and to general astonishment get their way."

Many believe, though, that the Communist party is far more entrenched in China than it was in eastern Europe, having welded its rule to a dynamic economy. If anything, many think, the authorities may be tightening their grip.

Or, as the authors of Charter 08 asked: "Where is China headed in the 21st century? Will it continue with 'modernisation' under authoritarian rule, or will it embrace universal human values, join the mainstream of civilised nations, and build a democratic system? There can be no avoiding these questions."